Photo-electric tube



April 1935. H. E. THOMSON 1,996,557

PHOTO-ELECTRIC TUBE Filed 001,- 23, 1931 Invent] or Hugh E. Thomson,

b5 @Www His Amer-ne Patented Apr. 2, 1935 PATENT OFFICE PHOTO-ELECTRIC 'rmm Hugh E. Thomson, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application October 23, 1931, Serial No. 570,884

11 Claims.

The present invention relates to light-sensitive apparatus and more particularly to photoelectric tubes of the gaseous type.

Photo-electric tubes may be divided broadly into two groups, depending upon the manner in which the photo-electrons move toward the positive electrode. In one group there may be placed those devices which operate on a pure electron basis as when the envelope is highly evacuated,

and the other group may consistof those devices which depend upon ionization between the electrodes in which case, theenvelope contains a definite amount of gas. Photo-electric tubes of the highly evacuated type are utilized in systems where uniformity of operation and rapidity of response are necessary, but they have the disadvantage of producing photo-currents of small magnitude calling for one or more stages of amplifying accessories in order to derive a current of practical amount.

On the other hand, while gas-filled tubes give photo-currents of a larger order than those of the pure electron discharge type, thus necessitating less amplification if any, they have the disadvantage of being relatively short-lived as compared to the highly evacuated type. It appears that the positive ions formed in the gas bombard the cathode and tend to destroy or at least to damage the light-sensitive surface. Moreover, the operation of a gas-filled tube is normally critical and unstable in the sense that when the applied potentials are increased accidentally or otherwise above a predetermined value, the current flowing through the tube may break into an are or glow.

An object of the present invention is to improve gas-fllled photo-electric tubes to the extent that the advantages of the gas may be obtained without necessitating undue positive bombardment of the cathode or the tendency to become unstable at excessive potentials. This object is attained, in brief, by providing within the tube, a positive ion collecting member which serves to divert the positive ions away from the cathode and hence, to protect the latter. This member also serves to endow the tube with increased stability of operation. The positive ion collecting member may take the form of two planar elec- 5 trodes connected in parallel and maintained substantially at cathode potential. The invention will be better understood when reference is made to the following description and accompanying drawing which depicts in perspective, partly broken away, a view of a photo- (c1. also-21.5)

electric tube improved in accordance with my invention.

In the drawing, numeral I designates a spherical envelope of the tipless" type so-called which is drawn down to a constricted portion surrounded by base 2. The envelope is provided with a reentrant stem 3 which serves to support the various electrodes. The cathode 4 is located centrally of the envelope and positioned in place by means of a rod upright 5 fused in the stem. 10 The cathode may constitute a rectangular plate of nickel or copper coated with light-sensitive material as will be explained hereinafter. A glass pant-leg 6 is provided about the portion of the conductor 5 which projects from the stem. 1 The anode I may consist of a flat wire helix of any suitable metal, such as nickel, closely surrounding the cathode and wound on a pair of parallel uprights which are secured to two rigid supports. These uprights are spaced a distance 30 apart somewhat greater than the width of the cathode member, and the helix extends in the vertical direction a distance considerably greater than the length of the cathode so that the latter is entirely surrounded by the anode member. 95 One of the vertical uprights about which the anode helix is wound may be extended through the stem to constitute a leading-in conductor l2.

If desired, the anode may be fabricated of mesh material as it is simply necessary to provide a 30 surface which is transparent to light. One of the vertical uprights of the anode frame may support a capsule member 8 the purpose of which will be explained hereinafter.

In addition to the cathode and anode and in accordance with my invention, the envelope also contains two electrodes 9 which are positioned preferably in planes, parallel to the cathode and anode and symmetrically arranged on each side of the latter at a considerable distance apart. These electrodes are positive ion collectors as will be pointed out hereinafter, and are maintained at the same potential by means of wire I!) and held rigidly in place by a framework II which terminates at the stem. The electrodes 9 may consist of rectangular frame members with fine wires of any suitable material, e. g. nickel strung between the vertical uprights of the frame. One of the rods which support the electrodes from the stem may be extended through the envelope to constitute a leading-in conductor l3.

The cathode 4 which may consist of a thin,- solid plate member is sensitized by first providing a metal coating thereon to which the light-sensitive material will tenaciously adhere. For this purpose, the cathode is silver-plated by any wellknown process; for example by electro-plating, chemical deposition or electrical sputtering, as disclosed in the Ruggles Patent No. 1,568,694. The silver coating may be oxidized, preparatory to receiving the light-sensitive material, by immersing the plate in an oxidizing atmosphere temporarily contained within the envelope I, and producing a glow between the anode and cathode by applying high voltage thereto from a spark coil or from a vacuum tube rectifier set. The depth of the oxide layer may be controlled by the duration and intensity of the electrical treatment. If desired, the plating and oxidation process may be carried out in an enclosure separate from the envelope 1.

When the desired degree of oxidation has been effected, the plate may be mounted in the envelope together with the grid-like anode 1. In order to sensitize the silver or other metallic oxide surface on the cathode, caesium or other suitable alkali vapor may be introduced in the envelope either by distillation from a side tube or preferably from a pellet contained within the capsule 8 and heated by high frequency currents in the well-known manner. The caesium vapor condenses on all of the electrodes within the tube but sticks only to the cathode surface inasmuch as the oxygen layer thereon serves as a binder or glue. The excess alkali material preferably is removed from the envelope in any suitable manner, for example, by distillation and evacuation of the excess vapor to a manifold, in order to lend.

uniformity of operation to the device. After the envelope has been highly evacuated, an inert gas such as argon-may be introduced at a pressure between 20 and 300 microns.

In operation, when a light beam is caused to impinge on the light-sensitive cathode through the grid-like or mesh anode, and a suitable potential applied to the cathode and anode, photocurrents are produced within the tube and flow through an external circuit connecting these electrodes. The normal function of the gas is to produce the requisite number of positive ions at the operating potentials to reduce space charge effect which normally inhibits the flow of current and thereby to increase the power output of the tube, in other words, to produce gas amplification. It has been found that the positive ions, particularly when generated in large numbers, as for example when the operating voltage is comparatively high, are attracted toward the cathode which they bombard vigorously; The lightsensitive surface is of delicate formation and under bombardment rapidly deteriorates in some manner, perhaps by destruction of the oxygen binder so that the tube is rendered less sensitive to light and becomes short-lived. It has further been found that the majority of the positive ions which find their way toward the cathode surface are formed in a region remote from the space bounded by the cathode and anode and that these positive ions together with those formed in the space between the cathode and anode return for the most part to the cathode, to add their bombarding effect. Migratory positive ions may also collide with the various elements within the tube including the envelope, and produce products which also impact with the cathode.

Regardless of the correct explanation for the decided impairment to the light-sensitive layer in a gas-filled photo-electric tube, I have discovered that if a negatively charged electrode be provided in a region outside of that bounded by the cathode and anode, the destructive action referred to is precluded or at least, substantially reduced. Thus, as shown in the figure, I provide the screen or grid-like member 9 transparent to light and positioned on each side of the anode. These members are advantageously located a considerable distance from the anode so as not to affect the field of the latter. While the members provide a shielding effect when charged to any positive potential less than that of the anode, I find that they function most efficiently when they are maintained at cathode potential. A permanent equi-potential connection between the members and the cathode may be made within the tube but preferably, separate leads are brought out in order that any difference of potential as desired, such as that indicated by the battery l4, may be maintained between the electrodes. As stated hereinbefore, the purpose of the members 9 is to attract and collect the positive ions formed in the space exterior to the anode and consequently, prevent these ions from finding also lend stability of operation to the device since a momentary accidental increase of potential does not cause undue ionization.

Whereas in the case of an ordinary gas-filled photo-electric tube, an amplification of approximately 10 may be obtained by the presence of gas, in a tube improved in accordance with my invention, i. e. provided with the positive ion collectors, an amplification of practically 100 is possible. This increase of amplification and hence, of translation efficiency, is brought about without any sacrifice in the life of the tube, since it has been pointed out that the cathode is no longer susceptible to excessive positive ion bombardment. 1

What I claim as new and desire to secure by Letters Patent of the United States, is:-

1. A photo-electric tube comprising an envelope containing inert gas and a plurality of cooperating electrodes including a member adapted tobe activated by light and sensitive to excessive positive ion bombardment, an electrical conducting means physically separate from said electrodes for collecting positive ions whereby the light-sensitive member is protected from excessive positive ion bombardment.

2. A photo-electric tube comprising an envelope containing an anode and a cathode adapted to be activated by light and sensitive to excessive positive ion bombardment, a source of positive ions in said envelope, and means for deflecting the excess of positive ions away from the cathode, said means comprising an electrode to be discharged negatively with respect to said anode.

3. A photo-electric tube comprising an envelope containing an anode and a cathode adapted to be activated by light and sensitive to excessive positive ion bombardment, a source of positive ions in said envelope, and means for deflecting the excess of positive ions away from the cathode, said means comprising an electrode to be maintained at cathode potential.

4. A photo-electric tube comprising an envelope containing an anode and a. light-sensitive cathode provided with an adsorbed layer of lightsensitive material, means for binding the layer to the cathode material, a source of positive ions in said envelope, and means for preventing disintegration or the binder by positive ion impact during operation of the device whereby the lightsensitive material is maintained on said cathode.

5. A photo-electric tube comprising an envelope containing inert gas and a plurality of electrodes including an anode and a light-sensitive cathode which tends to disintegrate when subjected to excessive positive ion bombardment, and means for preventing the said disintegration under operating conditions, said means including an electrically conducting member positioned remote from said electrodes and to be maintained at a potential less positive than said anode.

6. A photo-electric tube comprising an envelope containing inert gas and only three electrodes, to wit: a light-sensitive cathode, an anode positioned adjacent thereto and an electrode to be maintained at substantially cathode potential positioned on the opposite side of said anode from said cathode.

7. A photo-electric tube comprising an envelope containing inertgas and only-three electrodes, to wit: a light-sensitive cathode, an anode positioned adjacent thereto and an electrode to be maintained at substantially cathode potential positioned on the opposite side 01' said anode.

from said cathode, said anode and said electrode being transparent to light.

8. A photo-electric tube comprising an envelope containing inert gas and only three electrodes, to wit: a caesiated cathode, an anode, and an electrode cooperating with said anode for shielding the cathode from excessive positive ion bombardment during operation.

9. A photo-electric tube comprising an envelope containing argon, a plate-like caesiated cathode, an anode comprising a flat helix surrounding said cathode and a pair of plane grid-like electrodes, one positioned respectively on each side of the anode helix and adapted to be charged to cathode potential.

10. A photo-electric tube comprising an envelope containing inert gas, a light-sensitive cathode, an anode closely surrounding said cathode, and an electrode to be maintained at a potential less positive than said anode, the space between,

said cathode and said electrode being many times greater than the limited space between the cathode and the anode.

11. A photo-electric tube comprising an envelope containing inert gas, a light-sensitive cathode, an anode surrounding said cathode, and an electrode to be maintained at a potential less positive than said anode, said electrode being constituted of physically separate but electrically connected elements positioned respectively on opposite sides or the cathode.

HUGH E. THOMSON.

CERTIFICATE or CORRECTION.

PatentNo. 1,996,557. April z, 1935.

' HUGH E. THOMSON.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correctiorras follows: Page 2, second column, line 62, claim 2, strike out the syllable "dis-"; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 21st day of May, A. D. 1935.

Leslie Frazer (Seal) Acting Commissioner of Patents. 

